The present invention relates to an arrangement for understoichiometric gasification of spent liquor from chemical pulp production, comprising an upper reactor part, which upper reactor part is provided with a burner for the spent liquor and with an internally clad reactor jacket, and a lower separating part for separating a phase of solid and/or molten material, formed on gasification, from a phase of combustible gaseous material. The arrangement is principally intended for use in conjunction with the recovery of energy and chemicals from an expended cooking liquor from production of chemical paper pulp from a material containing lignocellulose.
For many years, the commercially dominant process for recovering energy and chemicals from so-called black liquor, which has been obtained in paper pulp production by the sulphate method, has generally been the so-called Tomlinson process, also called a recovery boiler.
A more modern process is described in Swedish Patent SE-C-448,173, which process is based on understoichiometric gasification/pyrolysis (i.e. with an oxygen deficit) of the black liquor in a reactor. The products are in this case a phase of solid and/or molten material, essentially comprising sodium carbonate, potassium hydroxide and sodium sulphide, and an energy-rich, combustible gas phase, essentially comprising carbon monoxide, carbon dioxide, methane, hydrogen gas and hydrogen sulphide. The mixture of solid/molten phase and gas phase is cooled and separated in a separating part connected to the reactor by means of direct contact with green liquor, the solid/molten phase dissolving in the green liquor. The green liquor is then conveyed for conventional causticizing for production of white liquor. The gas phase is used as fuel for generating steam and/or electrical power.
The reactor in the process/arrangement according to abovementioned SE-C-448,173 consists of a vessel which is internally clad/lined with a ceramic material. Other known reactors of the same construction type are shown, for example, in WO94/20677, WO93/02249 and WO93/24704. The separating part is usually arranged so that its outer walls constitute a continuation of the vessel for the reactor, the ceramic cladding of the reactor being arranged to present a cone-shaped constriction between the reactor and the separating part.
A problem with this known construction type for reactors is that in the event of uncontrolled expansion of the ceramic lining, for example caused by thermal expansion or diffusion of materials, these forces are transmitted to the walls of the pressure vessel. The ceramic lining has an ability to absorb inorganic material in the form of very small molten particles. These particles have been found to be able to reach through the reactor lining and then crystallize in an outer layer thereof, whereupon the lining is expanded. Uncontrolled expansion causes stresses in, and shortens the lifetime of, both the pressure vessel and the ceramic lining. Another problem is that changing the ceramic lining when it becomes worn is difficult because the ceramic material has to be removed without damaging the pressure vessel.
Further disadvantages/problems with the known construction type are heat losses from the reactor via the ceramic lining and the vessel walls, which losses are not used in the process.
CH 585,371 discloses a combustion arrangement or complete combustion of spent liquids, comprising a reactor part and a quench. The reactor part is in this case surrounded by an outer vessel, and there is a gap between the reactor part and the outer vessel, in which gap a coolant liquid is circulated in a closed circulation. The coolant liquid consists of a liquid which is entirely separate from the rest of the system, i.e. without direct contact with either gas phase or liquid phase in reactor and quench.
WO97/37944 also discloses a combustion arrangement for complete combustion of spent liquids, comprising a reactor part and a quench, and with a gap between the reactor part and an outer vessel. A coolant liquid is circulated in the gap, which coolant liquid is entirely separate from the rest of the system and is in part evaporated. It is also noted that the reactor in this embodiment is not lined.
The object of the present invention is to reduce or eliminate the abovementioned problems by making available an arrangement for understoichiometric gasification of spent liquor from chemical pulp production, where the reactor part constitutes an exchangeable prefabricated unit and where a favourable temperature profile in the reactor wall is obtained. At the same time, the heat losses are reduced and the heat transmission which takes place in the reactor part is utilized and increases the partial steam pressure of the water. This means that steam generation is improved, by about 5 to 10%, in subsequent condensation stages for gas cooling, which improves the economics of the process.
The arrangement according to the invention is defined in Patent claim 1.
According to the invention, the internally clad reactor part is surrounded by an outer vessel, with a gap between the said reactor jacket and the said outer vessel. Connected to the gap there are one or more inlets for a coolant medium, preferably a coolant liquid which preferably consists of a condensate which, in the process, is in direct contact with at least the gas phase of combustible material formed in the process.
As there is direct contact and direct connection between gas phase and condensate used as coolant medium, this means that essentially the same pressure is present inside the reactor as in the gap between the reactor and the outer vessel. By this means, the reactor part does not need to be designed as a pressure vessel, and it is sufficient for the outer vessel to be designed as a pressure vessel. The reactor part is thus comparatively simple and can be easily replaced on operational shutdown by means of the fact that the outer vessel can be opened so that the reactor part can be lifted out of this and replaced with a prefabricated unit.
In the gap, the temperature is preferably the saturation temperature at the prevailing pressure, the coolant liquid being partially evaporated. The heat taken up in the coolant liquid can be utilized for production of steam, preferably low-pressure or medium-pressure steam.
The upper reactor part with ceramic lining is connected to a lower separating part cooled by a film of liquid, in which separating part smelt and combustion gas are separated. However, a considerable number of the reactions also take place in the separating part, which affords an extended reaction space. However, in the following description, this lower part is referred to only with respect to the separating stage.
By virtue of the fact that the ceramic lining is cooled by the coolant liquid, a favourable temperature gradient is obtained between the inner surface of the reactor jacket and its outer surface, and inorganic material which has migrated into the ceramic will crystallize/freeze before it reaches the outer surface of the lining. A temperature corresponding to the freezing point of alkali will be present in the reactor lining at a predetermined depth.
By means of the continuous external sprinkling of coolant liquid, the positioning of this freezing point can be controlled in an optimum and controlled manner over the whole of the reactor vessel.
According to one aspect of the invention, the reactor operates at a pressure of 1.5-150 bar (abs.), preferably 1.5-50 bar, although atmospheric pressure is also conceivable. The temperature in the reactor can be 500-1600xc2x0 C., preferably 700-1300xc2x0 C., and the temperature gradient over the reactor jacket will thus range from the reactor temperature on the inside to the saturation temperature at the prevailing pressure.